The present invention relates generally to laser generated ion channels used for testing. More specifically, it relates to a laser generated ion channel that uses at least one laser to create an ion channel in a specially prepared shroud gas for testing electrical circuitry.
The printed circuit board (PCB) industry mandates the rapid testing of its electrical circuitry. As the size of electrical circuitry features, such as test pads, have become smaller due to higher circuit densities, traditional testing methods which rely on solid-to-solid contacts have become too slow, too destructive (from physical damage to circuitry features), and too costly (due to increased resolution and speed requirements). Thus, there is great need for developing new methods of xe2x80x9cnon-contactxe2x80x9d testing of PCBs that are faster and less destructive.
Non-contact testing of the prior art typically takes advantage of the photoelectric effect (photoelectric emission of electrons from the surface of a metal) to complete a test circuit. For example, U.S. Pat. Nos. 5,030,909, 4,706,018, and 4,644,264 all disclose the use of the photoelectric effect in conjunction with high intensity pulsed laser beams to facilitate noncontact testing. U.S. Pat. No. 5,030,909, to Blancha et al. discloses a contactless testing method that involves photoelectron emission from solid surfaces in an evacuated chamber using high power pulsed lasers emitting photons of optimum energy. U.S. Pat. No. 4,706,018 to Beha discloses a noncontact dynamic tester for integrated circuits that uses short pulses of high intensity laser light to generate photoelectron emission from solid surfaces in a high vacuum environment having a pressure of about 10xe2x88x925 to 10xe2x88x926 Torr. U.S. Pat. No. 4,644,264 to Beha et al. discloses photon assisted tunneling testing of passivated integrated circuits, a noncontact technique using high intensity lasers having short pulses of about 5 picoseconds to produce photoelectron emission from solid surfaces, such as test pads.
As each of the three patents described all operate using the scientific principle called the photoelectric effect they inherently suffer from operational defects including, but not limited to, the need for high intensity short pulse lasers, passivation layers, overlayers and/or vacuum systems for successful implementation.
In U.S. Pat. Nos. 5,345,465 and 5,335,238 and in the publication Laser Frequency Modulated Spectroscopy of a Laser-Guided Plasma in Sodium Vapor, J. MOL. SPECTROSC. 186, 222-229 (1997), the present inventors disclose non-contact apparati and methods for guiding an electric discharge which do not take advantage of the photoelectric effect. Such publications disclose, instead, employment of a single quasiresonant laser beam generated by photoexcitation of alkali atoms in the gas phase (so called, xe2x80x9cLaser Guided Dischargesxe2x80x9d or xe2x80x9cLGDxe2x80x9d). While eliciting several advantages over prior art techniques employing the photoelectric effect, quasiresonant photoexcitation suffers from the disadvantage of requiring an undesirable threshold density.
A need exists, therefore, for a non-contact method of testing PCBs that is faster, less destructive and less expensive than those methods currently available and which does not rely on the use of the photoelectric effect nor employ quasiresonant photoexcitation.
The present invention provides a noncontact method and apparatus for testing electrical circuitry which provides significant improvements in both resolution and speed over the prior art. The attributes of noncontact, high resolution, and speed may be satisfied by using inexpensive low intensity resonant laser beams in a novel shroud gas to create an electrically conductive ion channel microprobe.
The conductive ion channel microprobe of the present invention may be used to create an electrically conductive path between a circuit""s test pad or point and signal generation and detection apparatus. If the circuit""s test pad or point is functioning properly, then the ion channel microprobe will complete the electrically conductive path, the signal generation device will produce a signal over the conductive path and the signal detection device will detect or measure the signal. If the circuit""s test pad or point is malfunctioning, the conductive path will remain open and the signal detection device will not detect a signal.
The present invention operates using the principle of photoionization of a gas and not the photoelectric effect. Advantageously, the present invention can operate with low intensity continuous wave (CW) lasers without the need for use of passivation layers, overlayers, vacuum systems, or chambers of any kind.
Unlike the prior art, the laser of the present invention uses resonant excitation, which couples the laser field directly to the ground atomic state, as opposed to quasiresonant excitation which does not. By employing resonant excitation, the present system may advantageously operate using inexpensive low intensity resonant laser sources.
The present ion channel microprobe (ICM) invention proffers significant advantages over LGD-based systems, such as: permitting the use of a transparent semiconductor electrode instead of a hollow metal electrode; allowing operation at an unheated temperature of about 300 K (instead of about 800 K typically employed in LGD-based systems) and at about 760 Torr pressure (instead of about 1 Torr typically employed in LGD-based systems); and permitting measurements to be taken in an atmospheric environment not requiring a chamber (instead of requiring a vacuum chamber environment as in LGD-based systems). Also, fundamental differences exist with the laser beam dynamicsxe2x80x94the ICM may make. use of a scannable laser having high speed and resolution, while LGD conventionally employs a fixed laser having an unscannable configuration. The ICM may further make use of two or more ion channels for scanning conductivity tests of printed circuit board features. The ICM technique of the present invention differs from LGD in that it operates at typical alkali densities of about 1011 atoms per cm3, approximately six orders of magnitude below the quasiresonant threshold density of about 1017 atoms per cm3.
Generally, the system for producing an ion channel of the present invention comprises a first electrically conductive material such as a transparent electrode, a second electrically conductive material such as a test pad, a shroud gas containing a gas of either argon, helium, neon, krypton, xenon or nitrogen, and an alkali metal of either rubidium, resonant laser beam such that the laser beam propagates through the first conductive material, propagates through the shroud gas to produce an ion channel within the shroud gas, contacts the second conductive material, and creates an electrically conductive ion channel between the first electrically conductive material and the second electrically conductive material to define first and second terminals, respectively, of an electrical path.
Because the present invention uses resonant laser beams to produce photoionization, the present method of non-contact testing is less expensive, fast and has greater resolution.